@GreenGiant said:
Currently you can add new people to a group private message, but you can't remove them.

You would need to start a new conversation with the participants you want included. I don't believe there is a way to ignore the user just per conversation, and there is no way to prevent them from receiving messages if they are already included. The user themselves can delete themselves from the conversation and will no longer receive messages. However, If I recall correctly they could re-add themselves back in if they had a link.

Would also be good to title private message threads to easily identify different threads between the same group of users

Yes, this would allow you to add subjects when you start new messages, but at this point there is no way to add a subject to an existing message, except via adding the subject to the conversation table for the conversation id.
Conversely, IIRC If you set $Configuration['Conversations']['Subjects']['Visible'] = false; it will remove the ability to add a subject in the form when adding a mesage, but the Subject Title if already added prior to config change would appear as a title. Essentially the config add the form option, but doesn't turn subjects on or off, just the ability to add them when starting a new message.

But it might be a handy thing for the user who started the conversation to add a subject to older messages that have no subject, either as part of a core enhancement or via a plugin.

Pragmatism is all I have to offer. Avoiding the sidelines and providing centerline pro-tips.